Fourings,

Having copper cores is not enough. The gap needs to be closed up to .035. It is VERY easy to R&R plugs on this engine. It would be easy to simply pull them and gap them to .035 and replace them. It would be interesting to know the gap before resetting.

I think the book says .038, but it was learned a long time ago and discussed on this board about the idle advantage of a .035 gap on a copper core plug.

Good luck,